1) Field of the Invention
The present invention relates to a coated polyester film having utility as a substrate for magnetic tape applications such as video tape. More particularly, the present invention relates to a polyester film having an acrylic coating having a relatively high methyl methacrylate content on at least one major surface of said film. It has been found that relatively high methyl methacrylate content terpolymers, in combination with specific surfactants and using relatively low anionic surfactant concentrations, impart a "nodule" surface to polyester film substrates which reduced the coefficient of friction of such films.
2) Prior Art
Oriented polyester films possess generally good mechanical properties resulting in wide commercial acceptance as substrates for magnetic tape applications. The polyester film substrate may be coated on at least one side with an adhesion-promoting primer coating, which is itself overcoated with a magnetizable coating.
Acrylic terpolymers conforming to the terpolymers of the present invention have previously been incorporated in aqueous coating dispersions for polyester films. U.S. Pat. No. 4,371,489 to P. McGrail, titled "Production Of Anti-Static Thermoplastic Films" broadly discloses a methyl methacrylate/ethyl acrylate/methacrylamide terpolymer having molar range percentages of 40 to 80 percent methyl methacrylate; 15 to 50 mole percent ethyl acrylate; and 2 to 25 mole percent crosslinkable comonomer. The preferred methyl methacrylate molar range is taught to be 45 to 50 percent. The McGrail patent teaches the use of this terpolymer in combination with an alkyl or aryl phosphate ester which is present as free acid or as a partial or complete sodium or potassium salt to form a coating composition for polyester films. The McGrail patent does not disclose nor suggest the creation of a nodule surface as a method of reducing the high coefficient of friction of polyester film.
Similarly, "Polyester Film Primed With Crosslinked Acrylic Polymers", Japanese Patent Application No. 259422/1984 (Published Aug. 6, 1985), discloses a methyl methacrylate/ethyl acrylate/N-methylol acrylamide terpolymer wherein the methyl methacrylate preferably comprises 35 to 85 mole percent of the terpolymer. The application broadly suggests the use of a surfactant, but does not disclose specific surfactants for use with the acrylic terpolymer.
Other patents also disclose crosslinkable acrylic terpolymers as having utility as coatings for polyester films, typically to reduce static charge build-up on the film as it travels over record/playback heads, rollers, guidepins, and other components of electronic equipment. Generally, however, these patents teach the use of relatively lower amounts of methyl methacrylate. For example, U.S. Pat. No. 4,214,035 (July 22, 1980) to J. Heberger, titled "Antistatic Coated Polyester Films", (commonly assigned) broadly discloses a methyl methacrylate/ethyl acrylate/methacrylamide terpolymer having a glass transition temperature in the range of from 40.degree. to 50.degree. C., preferably 45.degree. C. While the patent is silent as to any mole percentages of the components of the terpolymer, it is believed that the methyl methacrylate molar percentage of this terpolymer does not exceed 50 mole percent.
Similarly, U.S. Pat. No. 3,819,773 (June 25, 1974) to G. Pears, titled "Method For Preparing Reclaimable Coated Polyester Films", discloses a crosslinkable acrylic terpolymer comprising 50 percent by weight methyl methacrylate.
British Patent Specification No. 1,125,460, "Improvements Relating To Polyester Film Base" (Published August 28, 1968), discloses a high (70 to 95 percent by weight) methyl acrylate (as opposed to methyl methacrylate) terpolymer, but it comprises itaconic acid (2 to 20 percent) and either diallyl phthlate or divinylbenzene (3 to 30 percent) as the other comonomers rather than the comonomers of the present invention. The patent is directed to rendering polyethylene terephthalate film less hydrophobic, and does not disclose nor suggest the creation of a "nodule" surface as a method of reducing the high coefficient of friction of polyester film.
"A Process For Preparing Subbing Coated Polyester Film", British Patent Specification No. 1,168,171 (Published Oct. 22, 1969), discloses high (70 to 90 percent by weight) content methyl methacrylate terpolymers which must have 3 to 30 percent of itaconic acid, and 7 to 20 percent of an aliphatic poly-functional compound containing at least two sites of carbon to carbon unsaturation, where at least one of the sites is represented by an allylic or vinylic group, glycidyl acrylate, glycidyl methacrylate esters wherein the alcohol portion of the ester is hydroxy substituted, or fumaric acid.
U.S. Pat. No. 4,571,363 to Culbertson et al (commonly assigned) teaches a polyester film primed with an acrylic, crosslinked terpolymer for a primer coating. The primer coating comprises at least about 50 percent by weight of acrylic and/or methacrylic monomers; from about 1 percent to about 15 percent by weight of a comonomer capable of inter-molecular crosslinking upon the application of heat, and from 0 to about 49 percent by weight of one or more halogen-free mono-ethylenically unsaturated monomers copolymerizable therewith. While this patent states that it is desirable to include a surfactant for wettability, no specific surfactant types and amounts are disclosed or required.
One problem associated with polyester film is the high coefficient of friction of uncoated and unmodified oriented polyester film. Such film typically is very smooth, and tends to "block" or stick to other layers of such film when wound upon itself.
Prior artisans have devised two methods for reducing the high coefficient of friction of oriented polyester film, both of which involve roughening the smooth surface of the polyester film in order to reduce its high coefficient of friction. The polyester film surface must not be excessively roughened, or its utility as a substrate for magnetic tape applications will be destroyed. One well known method involves coating the film with a crosslinkable coating which contains particles and then crosslinking the coating composition, thereby binding the particles. The function of the particles is to reduce the coefficient of friction by creating a more roughened surface than the uncoated polyester film.
Coating the polyester film surface with a particle-containing coating composition to reduce the film's coefficient of friction is not a panacea. Problems which are associated with this method include wide particle size distribution and agglomeration of the particles, both of which tend to degrade the film's utility as a magnetic tape substrate.
Another method for reducing the inherently high coefficient of friction of polyester film involves the creation of "internal" particles during the polyester synthesis by controlled precipitation of the catalyst residue and reaction thereof with polyester monomers or oligomers. Such particles are substantially homogeneously distributed throughout the polyester resin, and only a small amount are present at or near the surface of the polyester film where coefficient of friction is measured. Such internally generated particles are described more fully in U.S. Pat. Nos. 4,138,386 and 4,067,855. This method requires extremely close control over the polyester synthesis and can create processing difficulties during the manufacture of polyester film from such resin.
U.S. Pat. No. 4,138,386 (Feb. 6, 1979) to M. Motegi et al, titled "Polyester Film For Magnetic Tape", discloses the use of internally-generated polyester particles in combination with inert particles selected from the group consisting of oxides or salts of elements of Groups II, III, or IV of the periodic table.
U.S. Pat. No. 4,461,797 (July 24, 1984) to T. Adachi et al, titled "Polyester Film With Projections And Depressions On The Surface", discloses the addition of organic or inorganic particles to a polyester resin prior to manufacture into a polyester film. By preheating the extruded polyester film prior to orientation, void formation around the particles is substantially eliminated. The surface of the Adachi et al '797 biaxially oriented polyester film is characterized by elliptical depressions having a projection in the middle of the depression. This surface exhibits a reduced coefficient of friction in comparison to polyester film which was not preheated prior to orientation.
U.S. Pat. No. 4,233,352 (Nov. 11, 1980) to M. Ono et al, titled "Polyester Film And Method Of Manufacture Thereof", discloses a polyester film having a discontinuous layer of a water soluble polymer on at least one major surface. The discontinuous polymeric layer comprises discrete "worm-like" patterns comprising a mixture of a water soluble polymer, polysiloxane and/or styrene-butadiene polymer, and optionally natural wax. The water soluble polymer may be any water soluble polymer having a molecular weight between 10,000 and 2,000,000. Suitable water soluble polymers include methyl cellulose, hydroxyethyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, gum tragacanth, gum arabic, casein, and gelatin.
U.S. Pat. No. 4,548,855 (Oct. 22, 1985) to M. Ono et al, titled "Polyester Film For Magnetic Recording Medium", is an improvement over the film disclosed in Ono et al '352. Ono et al '855 discloses a polyester film having a discontinuous layer of a water soluble polymer on at least one major surface. The discontinuous layer should not exceed 500 angstroms in thickness, and is preferably from 50 to 300 angstroms thick. The water soluble polymer should have a molecular weight of from 10,000 to 2,000,000, and may be polyvinyl alcohol, tragacanth gum, gum arabic, casein, gelatin, methyl cellulose, hydroxyethyl cellulose, and carboxymethyl cellulose. Ono et al '855 further discloses fine particles which are present on the surface of the discontinuous layer and the exposed polyester film surface. The fine particles may be alkali metal salt derivatives of sulfonic acid which are mixed with the polyester resin or the particles may be "internally" generated by precipitation of the polyester catalyst. The particle size may range from 50 to 400 angstroms, and the shape of the particles is not critical.